Adaptive encoding of orthogonal transform coefficients in the discrete Fourier transform (DFT), modified discrete cosine transform (MDCT), and the like is a known method of encoding speech signals and acoustic signals having a low bit rate (about 10 to 20 kbit/s, for example). A standard technique AMR-WB+ (extended adaptive multi-rate wideband), for example, has a transform coded excitation (TCX) encoding mode, in which DFT coefficients are normalized and vector-quantized in units of eight samples (refer to Non-patent literature 1, for example).